The invention relates to a polarization splitter. It also relates to a method for producing such a polarization splitter. It also relates to ophthalmic lenses having inserts for projecting an image towards the user, that include such a polarization splitter. The invention finally relates to devices for projecting an image towards a user, comprising such ophthalmic lenses.
Polarization splitters are optical components for splitting light into its various polarization components. The direction of light polarization is defined with respect to an oscillatory plane of the electric field. Most frequently, unpolarized light is split into two orthogonal linear polarizations. Here, we distinguish S (perpendicular) polarization and P (parallel) polarization. In S-polarized light, the plane of oscillation is perpendicular to the plane of incidence defined by the surface normal and incidence vector. In P-polarized light, the plane of oscillation is parallel to the plane of incidence. The components can be separated by absorption or by reflection.
Polarization splitting by reflection is based on the principle of total transmission of S-polarized light. We know that a ray striking an optical component at the Brewster angle is split into a polarized totally reflected ray and a partially polarized refracted ray of orthogonal polarization. The reflected and transmitted rays are at right angles to each other.
The Brewster angle depends on the refractive index of the two media making up the optical component. As refractive index varies with wavelength, the Brewster angle also depends on the wavelength of the light. Last, the angle of incidence for a ray can only correspond to the Brewster angle for one particular wavelength. Consequently, such a polarizer is only fully effective for one given wavelength and is not suitable for polarization splitting in a suitable manner over the whole spectrum, notably the visible spectrum.
The polarization splitter disclosed in U.S. Pat. No. 5,400,179 has a splitting ratio that is substantially constant over wavelengths greater than 720 nm, in other words in the near infrared. It is made up by a stack of layers of materials including praseodymium oxide Pr6O11, with a refractive index in three different ranges.
Also, stacks of thin films of materials having different refractive indices on a substrate forming anti-reflective coatings are known.
U.S. Pat. No. 6,313,577 discloses an anti-reflective coating carrying layers of praseodymium titanate. However, such a stack is not suitable for polarization splitting as the number of layers and their thicknesses are not adapted for this purpose. The anti-reflective treatment is optimized for normal incidence, contrary to reflective polarization separation treatment, which only operates for oblique angles of incidence.
The known types of splitter have however proved to be unsuitable for applications such as ophthalmic lenses having inserts for projecting an image towards the user.
By the term ophthalmic lenses we here mean systems for combining images for spectacles or head-mounted devices; an image is projected towards the wearer's eye by an optical path provided in the lens; here, the term “lens” means the optical system containing inserts notably designed to be mounted in a spectacle frame or a head-mounted device. The inserts can comprise mirrors, semi-reflecting plates, polarization splitting cubes, quarter wave plates, lenses, mirrors, concave reflecting lenses (a Mangin mirror for example), diffracting lenses and/or holographic components. A device for projecting images towards the user then comprises the lens mounted in spectacles or a head-mounted device and an image source such as a liquid crystal display, more particularly a micro-display.
In such applications, the polarization splitting elements process the polarized light delivered by the display elements currently employed in such micro-displays. The effectiveness of the polarization splitter will determine contrast and loss of image intensity and, consequently, the brightness of the image, thereby constituting a primordial factor.
The choice of certain polymers as the substrate for the ophthalmic lens constitutes one additional constraint on the specifications. Indeed, certain polymers must undergo thermal treatment at temperatures greater than 100° C. After this type of treatment, the appearance of cracking or crazing is frequently noticed at the stack interfaces.